If you have a electronic balance then simply weighing will do.
D2O is 1/18 or 5% heavier than water. If you have 1L of water it weighs 1000g. 1% of D2O will add another 0.2 g. Most scales can measure 1mg precision.
If you want ratio of DHO then the same can also be done, you just need to solve a few simultaneous equations:
a(16)+b(17)+c(18) = weight
a+b+c = 1L